MOSCOW, Jan. 22 -- Russian prosecutors and election officials said Tuesday that they are examining the alleged forging of signatures by the campaign of former prime minister Mikhail Kasyanov on his petition to get on the presidential ballot. The probe could lead to the opposition figure being disqualified from the March 2 vote.

Kasyanov, who became a fierce critic of Vladimir Putin after being fired by the president in 2004, was required to amass 2 million signatures because he was not nominated by a political party with seats in parliament.

Kasyanov said law enforcement agencies are pressuring people to say they did not sign his petition or forcing those who collected signatures to say they forged names.

In a statement, Kasyanov said his supporters had been "threatened with home searches and dismissal from their jobs." In several cases, he added, "people were threatened with being taken to the police by force and even arrested. In actual fact, they are being forced to denounce themselves."

The Central Election Commission said Tuesday that it had randomly checked 400,000 of the signatures collected by Kasyanov's campaign and found that 15.57 percent were fake.

"In line with current law, if the number of fake signatures exceeds 5 percent in the first check, another 10 percent of signatures are selected for another check," commission secretary Nikolai Konkin said in a statement.

If more than 5 percent of the signatures in the second sample are found to be fake, Kasyanov will be disqualified. The commission said it will announce its decision Sunday.

Separately, a spokeswoman for the Russian prosecutor's office said that a criminal investigation was opened after evidence of forgery by Kasyanov's campaign was found in two Russian administrative regions.

Kasyanov barely registers in opinion polls here and represents no political threat to the overwhelming favorite, Dmitry Medvedev, who holds the post of first deputy prime minister and is endorsed by Putin. But as a registered candidate for president, Kasyanov would enjoy media access that he has been denied here for several years. His supporters and some political analysts said the Kremlin may want to avoid giving him any kind of platform.

Medvedev, 42, was officially registered as a candidate for president on Monday. He launched his campaign Tuesday with a promise to fight corruption. "Russia is a country of legal nihilism" at a level "no European country can boast of," he said in a speech. "Corruption in official structures is at a huge scale, and the fight against it should be a national program."

Two other candidates, Communist Party leader Gennady Zyuganov and ultranationalist Vladimir Zhirinovsky, were nominated by political parties represented in parliament and therefore did not have to gather signatures.

A fifth potential candidate, Andrei Bogdanov, head of the small Democratic Party, is being backed by the Kremlin to broaden the field, according to political analysts here. Bogdanov, whose party drew only about 90,000 votes in parliamentary elections last month, also had to gather 2 million signatures.

So far, the election commission has reported no major problem with the signatures on Bogdanov's petition.

An opinion poll released Tuesday by the All-Russian Public Opinion Research Center found that of people planning to vote, 60 percent intend to support Medvedev. That figure was followed by 7.5 percent for Zhirinovsky, 6.1 percent for Zyuganov, 0.8 percent for Kasyanov and 0.2 percent for Bogdanov.